Greg, Dan: One of my first antennas put up in Alaska was a simple 2m J-Pole. I used the upper portion of a TV mast and used a 1/4 wave section of copper tubing clamped 3/4 wave length below the top end of the mast. Attaching coax to the stub and mast at a point to give a 50-ohm match. Worked fine. Note that the longer part of the J-Pole continued below where the 1/4 wave stub attached for about ten feet. There was no matching problems. What affect on the radiation pattern resulted is unknown, but I observed no detrimental effects.
Adding ground radials to the bottom of the J-pole should not have a big effect on tuning. I think the only effect on radiation is that you establish a better ground for the vertical 1/2 wave dipole (perhaps lowering the pattern a little closer to the horizon. I built a 6m/10m J-pole and it is mounted at ground level attached to a short tower set into the ground. The tower seems to not affect the tuning point. This summer I may try adding some 1/4 wave radials to the base to see if it has any affect. I set it up mainly for working 6m/10m FM, but is also usable for mode-A satellites. I have compared it to my 3-element triband yagi on A07 mode-A and the beam is better. http://www.kl7uw.com/J-Pole.htm Sidenote: I have re-installed satellite antennas for 2m, 70cm , and 13cm on my short tower this weekend. Both 2m and 70cm antennas working well. I have not installed the 2.4 GHz LNA or downconverters, as yet. Control wiring for the satellite antennas has not been connected plus the B5400 az-el rotator control unit still requires repair so antennas not rotatable. http://www.kl7uw.com/sat.htm 73, Ed - KL7UW At 09:49 PM 4/17/2011, Greg D. wrote: >Hi Dan, > > On 17/04/2011, at 15:53, KF1BUZ <kf1...@gmail.com> wrote: > > > A Copper Jpole, has this been tried? > > Just thinking it might make my getting into the birds better. > > > > Thanks > > Dan > > KF1BUZ > > > >If I interpret this right (subject + message), you are asking if >adding a Ground Plane to a J-pole antenna will improve its >performance in a satellite ground station application. I believe >the answer is no, though someone would need to do the modeling to >understand for sure. > >A J-pole is an end-fed dipole, with the "J" portion being a 1/4 wave >long at the frequency of the antenna. Recalling some RF theory >stuff, a 1/4 wave "matching section" has a low impedance on one end, >and a high impedance at the other. One end goes to the coax feed >line (low impedance), and the other is attached to the end (high >impedance) part of the dipole. You will find that the single pipe >section of the J-pole antenna is about a 1/2 wave long at the >antenna's design frequency, and since it's connected directly to the >end of the matching section, it makes for an end-fed dipole. Some >designs use a 5/8 wave dipole section for a little extra gain >towards the horizon. > >So a J-pole antenna is actually a pretty effective satellite >antenna, similar to a simple ground plane antenna but mechanically >more robust. I've used both kinds. My very first satellite contact >ion 1993 was using one for the uplink into RS-10, and that contact >was followed by many many more. That antenna is still in service >nearly 18 years later. (If you hear or use the KO6TH APRS iGate, >you're using it!) I've got two SO-239-type Ground Plane antenna >carcases in the garage; they didn't last. As a satellite antenna, >both Ground Plane and J-pole antennas do have a null directly >overhead, but very few satellite passes go directly overhead, and >when they do, they spend very little time there. So don't sweat it. > >A J-pole with a 1/2 wave section on top will also work as a dual >band 2M / 70CM antenna, with the upper band on the antenna's 3rd >harmonic. Using the antenna that way, I'm told, it has a radiation >pattern that is lifted somewhat from the horizon, so it should be a >good match for satellite work. > >But, back to your question... The J-pole antenna is a totally >balanced system in itself, and doesn't suffer for not having a >ground plane below it. What putting a ground plane some distance >below the end of an end-fed dipole will do to the radiation pattern, >however, is a modeling task for someone at a higher mental pay-grade >than me. My guess is that it will depend very significantly exactly >where the ground plane is mounted. You could alter both the >radiation pattern and the feed point impedance with that addition, >and maybe make things worse. > >Hope this helps a little, > >Greg KO6TH > > >_______________________________________________ >Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. >Not an AMSAT-NA member? Join now to support the amateur satellite program! >Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb 73, Ed - KL7UW, WD2XSH/45 ====================================== BP40IQ 500 KHz - 10-GHz www.kl7uw.com EME: 144-1.4kw, 432-100w, 1296-testing*, 3400-winter? DUBUS Magazine USA Rep dubus...@hotmail.com ====================================== _______________________________________________ Sent via AMSAT-BB@amsat.org. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
